This invention relates to a method of processing a polycondensate, in particular polyethylene terephthalate (PET), to yield granules formed as pellets, the polycondensate being melted in the course of the process and later solidified again.
Such methods, in particular for processing PET, are known. The PET starting material originates directly from PET synthesis, or recycled PET bottles (RPET) are used, in which case the RPET fraction coming from the bottle wall is preferred, so that one is working mainly with chips and/or flakes. These chips always contain, in addition to other unwanted impurities, small amounts of water, which may be residual water from the bottle washing process that has not been expelled and/or water that has entered during storage and shipping of the chips. If these chips of RPET or new PET having a water content are melted in an extruder to produce granules, the condensation reaction proceeds mainly in the opposite direction (establishing an equilibrium), and hydrolysis is predominant, so that there is a reduction in the average chain length, i.e., the degree of polymerization, so the viscosity (given as the viscosity index or the intrinsic viscosity IV) of the polymer also declines. This hydrolytic degradation is more pronounced, the greater the amount of water and time available for the degradation reaction.
Previous efforts in the state of the art have been aimed at reducing the water content. The goal in the usual predrying is to achieve a low water content of less than 100 ppm (typically approximately 30 ppm) to prevent an excessive reduction in the IV in the subsequent melting of the PET and/or RPET during the usual dwell times in the molten state, where the polycondensation equilibrium achieves significant reaction rates (above approx. 180-190° C. for PET). A reduction in IV by approx. 0.05 from approx. 0.80 to approx. 0.75 would be acceptable. This extensive predrying takes a relatively great amount of time and energy and retards the processing when continuous operation with integrated predrying is the goal.